Electrochemical capacitors

Historical introduction

Electrochemical capacitors provide a mode of electrical charge- and energy-storage and delivery, complementary to that by batteries. The first electrochemical capacitor device was disclosed in a General Electric Co. patent in 1957 to Becker but was of a crude nature, employing porous carbon. Later work by Sohio (1969) described a so-called "electrokinetic capacitor" utilizing porous carbon in a non-aqueous electrolyte which enabled it to be charged up to about 3 V, though the operation of the device was not "electrokinetic" in nature, a misnomer. In 1971, Trasatti and Buzzanca recognized that the electrochemical charging behavior of ruthenium dioxide films was like that of capacitors. Between 1975 and 1980, the present author and his co-workers, under contract with the then Continental Group Inc., carried out extensive fundamental and development work on the ruthenium oxide type of electrochemical capacitor (Conway, 1997) which behaves as a surface- redox pseudocapacitance (see below). The whole field has burgeoned since about 1990 and is very active in fundamental, and R&D directions, Above about the Electrochemical capacitors information content.

A great deal of scientific and technological research has been reported in the scientific literature since about 1990. An extensive and detailed account of this has been given in the author's monograph on "Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications" (1999), Above about the Electrochemical capacitors information content.

Scientific introduction

In order to describe "electrochemical capacitors" and to explain their function and applications, it is necessary first to consider the nature of an ordinary electrostatic capacitor or a "condenser" as it used to be called, and thence the meaning of the term electrical capacitance, Above about the Electrochemical capacitors information content.

The nature of electricity took a long time to be understood, from the early experiments on electrostatic electricity in the mid-18th century, for example by Galvani, through the time of the invention of the first electric battery by Alessandro Volta (Volta's "Pile") in 1800, on to Faradays's and Davy's monumental discoveries on the chemical origin of electricity generated by Volta's pile. At first, two "kinds" of electricity were postulated: "animal electricity", as in the works of Galvani on stimulation of the frog's leg nerve by contact between two dissimilar metals and later, "Voltaic electricity" generated chemically from a Volta pile of zinc and silver or copper plates separated by paper wetted with an acid or salt solution (Conway, 2000), Above about the Electrochemical capacitors information content.

In parallel with these discoveries were extensive works on electrostatic electricity generated for example by the rubbing of naturally occurring amber or by the so-called Wimshurst machine (a rotating circular plate, containing insets of amber-like material, rubbing against charge-collector plates connected overall to a Leyden Jar or a spark-gap). It was from this direction of research on electricity that the invention of the electric condenser arose, referred to as the "Leyden Jar", and capable of storing electric charge generated by a Wimshurst machine. Such a jar had the "capacity", depending on its dimensions and materials of construction, of storing electric charge by bringing it together in a condensed way (hence the term "condenser") on the surfaces of a Leyden Jar at a certain two-dimensional charge density, Above about the Electrochemical capacitors information content.

The principle of design and operation of the Leyden Jar and all subsequent regular condensers or capacitor devices, is as follows. Two metal surfaces that constitute electrodes are separated at some small distance either in air (or vacuum) or on each side of a liquid or solid film, referred to as the "dielectric", a term first used by Michael Faraday . For a given separation of the electrode plates, the capacitance developed per unit area of the two plates depends on the properties of the dielectric between the plates characterized by its so-called dielectric constant, Above about the Electrochemical capacitors information content.

In the case of the Leyden Jar (Figure 1), the material (glass) of the jar itself serves a the dielectric medium and the contact plates were metal foils wrapped, inside and out, around the cylindrical surfaces of the jar. The electrical contact to the inner surface foil was by means of a conducting electrolyte solution (or originally by ordinary water itself) in which was immersed a conducting metal electrode for electrical contact. The device was charged by joining two wires from the inside electrode and the outside foil to an electrostatic machine of the Wimshurst type. In later experimentation, the Leyden Jar capacitor was connected to the electrodes of a Volta's pile or battery for charging. This was the first-generation capacitor for storage of electric charge, Above about the Electrochemical capacitors information content.

The nature of electric charge remained elusive until much later (1897) when J.J. Thomson identified and characterized the fundamental entity of electric charge as the "electron", present ubiquitously in all atoms of the Universe and identified, in his experiments, by means of experiments on gases at low pressures in gas-discharge tubes (Crookes tubes or neon lights). The electron charge was determined independently by Townsend and by Millikan (Glasstone, 1940), and was shown to be equivalent to Faraday's constant for the relation between extent of passage of charge and extent of chemical change (as related by Faraday's Laws) caused by electrolysis of conducting solutions, when calculated on a "per gram-atom" or "gram-equivalent" basis, Above about the Electrochemical capacitors information content.

Relation of capacitance to geometry and dielectric constant of a capacitor

 In order to describe "electrochemical capacitors" and to explain their function and applications, it is necessary first to consider the nature of an ordinary electrostatic capacitor or a "condenser" as it used to be called, and thence the meaning of the term electrical capacitance.

The nature of electricity took a long time to be understood, from the early experiments on electrostatic electricity in the mid-18th century, for example by Galvani, through the time of the invention of the first electric battery by Alessandro Volta (Volta's "Pile") in 1800, on to Faradays's and Davy's monumental discoveries on the chemical origin of electricity generated by Volta's pile. At first, two "kinds" of electricity were postulated: "animal electricity", as in the works of Galvani on stimulation of the frog's leg nerve by contact between two dissimilar metals and later, "Voltaic electricity" generated chemically from a Volta pile of zinc and silver or copper plates separated by paper wetted with an acid or salt solution (Conway, 2000), Above about the Electrochemical capacitors information content.

In parallel with these discoveries were extensive works on electrostatic electricity generated for example by the rubbing of naturally occurring amber or by the so-called Wimshurst machine (a rotating circular plate, containing insets of amber-like material, rubbing against charge-collector plates connected overall to a Leyden Jar or a spark-gap). It was from this direction of research on electricity that the invention of the electric condenser arose, referred to as the "Leyden Jar", and capable of storing electric charge generated by a Wimshurst machine. Such a jar had the "capacity", depending on its dimensions and materials of construction, of storing electric charge by bringing it together in a condensed way (hence the term "condenser") on the surfaces of a Leyden Jar at a certain two-dimensional charge density, Above about the Electrochemical capacitors information content.

The principle of design and operation of the Leyden Jar and all subsequent regular condensers or capacitor devices, is as follows. Two metal surfaces that constitute electrodes are separated at some small distance either in air (or vacuum) or on each side of a liquid or solid film, referred to as the "dielectric", a term first used by Michael Faraday . For a given separation of the electrode plates, the capacitance developed per unit area of the two plates depends on the properties of the dielectric between the plates characterized by its so-called dielectric constant, Above about the Electrochemical capacitors information content.

In the case of the Leyden Jar (Figure 1), the material (glass) of the jar itself serves a the dielectric medium and the contact plates were metal foils wrapped, inside and out, around the cylindrical surfaces of the jar. The electrical contact to the inner surface foil was by means of a conducting electrolyte solution (or originally by ordinary water itself) in which was immersed a conducting metal electrode for electrical contact. The device was charged by joining two wires from the inside electrode and the outside foil to an electrostatic machine of the Wimshurst type. In later experimentation, the Leyden Jar capacitor was connected to the electrodes of a Volta's pile or battery for charging. This was the first-generation capacitor for storage of electric charge, Above about the Electrochemical capacitors information content.

The nature of electric charge remained elusive until much later (1897) when J.J. Thomson identified and characterized the fundamental entity of electric charge as the "electron", present ubiquitously in all atoms of the Universe and identified, in his experiments, by means of experiments on gases at low pressures in gas-discharge tubes (Crookes tubes or neon lights). The electron charge was determined independently by Townsend and by Millikan (Glasstone, 1940), and was shown to be equivalent to Faraday's constant for the relation between extent of passage of charge and extent of chemical change (as related by Faraday's Laws) caused by electrolysis of conducting solutions, when calculated on a "per gram-atom" or "gram-equivalent" basis, Above about the Electrochemical capacitors information content.

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